Optocoupler systems include a first circuit and a second circuit that are electrically isolated from each other. The first circuit includes a light emitting diode (LED) that is coupled to a LED current source. The first circuit is optically coupled to a second circuit. The second circuit includes a photodiode (PD). For example, the LED emits light, which impinges on the photodiode, causing a current through the photodiode (e.g., a photodiode current). The second circuit also includes a transimpedance amplifier circuit is coupled to the photodiode to generate an output voltage signal that is based on the photodiode current. The second circuit also includes a current source that generates a reference current. Typically, the photodiode current is compared with the reference signal, and this comparison is utilized to generate the output voltage signal.
Although the reference current is typically not dependent on temperature (i.e., relatively constant across temperature differences), the photodiode current changes or varies with respect to temperature. This temperature dependence causes the following unwanted and undesirable traits or attributes to the output voltage signal: 1) pulse width variation at different temperatures, and 2) pulse width distortion across temperature.
FIG. 6 illustrates several waveforms that represent various signals generated by a prior art optocoupler system, where the pulse width of the output voltage signals varies across different temperatures. It is noted that a first waveform 610 represents a reference current that is relatively fixed across temperatures.
A first waveform 620, a second waveform 630, and a third waveform 630 represent a photodiode current at different temperatures (e.g., cold temperature, room temperature, and hot temperature). An exemplary temperature range is from −40 degrees Celsius to +125 degrees Celsius. For example, the second waveform 620 represents the photodiode current signal at cold temperature (e.g., −40 degrees Celsius). The third waveform 630 represents the photodiode current signal at room temperature. The fourth waveform 640 represents the photodiode current signal at hot temperature (e.g., +125 degrees Celsius).
A fifth waveform 650, a sixth waveform 660, and a seventh waveform 670 represent output voltage signals generated by the prior art opto-coupler system at different operating temperatures. For example, the fifth waveform 650 represents the output voltage signal at room temperature. The sixth waveform 660 represents the output voltage signal at cold temperature (e.g., −40 degrees Celsius). The seventh waveform 670 represents the output voltage signal at hot temperature (e.g., +125 degrees Celsius).
As can be appreciated, the pulse width of each of the output voltage signal waveforms 650, 660, 670 is different and dependent upon temperature. It is noted that the propagation delay from off-state to on-state and on-state to off-state can be different due to asymmetric triggering at cold temperature and at hot temperature. The different propagation delays further causes pulse width distortion across the entire temperature range.
Based on the foregoing, there remains a need for an apparatus and method for generating an output signal that tracks the temperature coefficient of a light source that overcomes the disadvantages set forth previously.